1. Field of the Invention
The present invention relates to a self-contained closed-circuit oxygen-generating breathing apparatus, and more particularly to a breathing apparatus of this kind which is provided with a breathing bag capable of cooling oxygen-enriched gas to a temperature suitable for inhalation by the user.
2. Description of the Related Art
Hitherto, oxygen masks connected to oxygen cylinders have generally been used as breathing apparatuses in places such as mines, factories and similar sites, and scenes of fires where there is a risk that people may be adversely affected by inhaling air having a low oxygen concentration, poisonous gases, or vapor. However, since these oxygen masks are heavy and difficult to carry, self-contained closed-circuit oxygen-generating breathing apparatuses have recently been developed. Such a breathing apparatus employs as an oxygen-generating agent a substance, such as potassium superoxide (KO.sub.2) or sodium peroxide (Na.sub.2 O.sub.2), which generates oxygen by reacting with the carbon dioxide and moisture contained in exhalation gas.
When the oxygen-generating agent is, for instance, potassium superoxide, it is known that oxygen is generated, in general, by the following chemical reactions: EQU 2KO.sub.2 +H.sub.2 O.fwdarw.2KOH+3/2(O.sub.2) EQU 2KOH+CO.sub.2 .fwdarw.K.sub.2 CO.sub.3 +H.sub.2 O EQU 2KO.sub.2 +CO.sub.2 .fwdarw.K.sub.2 CO.sub.3 +3/2(O.sub.2)
All these reactions are exothermic reactions. The amount of heat generated in the reaction between KO.sub.2 and water is estimated to be about 6.6 kcal/mol KO.sub.2, while that generated in the reaction between KO.sub.2 and CO.sub.2 is estimated to be about 21.5 kcal/mol KO.sub.2.
Various types of self-contained closed-circuit oxygen-generating breathing apparatuses employing an oxygen-generating agent which generates oxygen by reacting with the carbon dioxide and moisture contained in exhalation gas have been developed. A well-known type is the one which mainly comprises a face piece, an exhalation gas tube, a breathing bag, and a canister of an oxygen-generating agent.
Breathing apparatuses of this type are disclosed in, for instance, Japanese Utility Model Publication Nos. 30873/1976, 30504/1980, and Japanese Utility Model Laid-Open No. 2256/1986.
In such a breathing apparatus, exhalation gas exhaled by the user is introduced from the face piece to the oxygen-generating agent canister through the exhalation gas tube, and, in the canister, oxygen is generated by reactions between an oxygen-generating agent and the carbon dioxide and moisture contained in the exhalation gas, and the resulting gas enriched in oxygen passes through the breathing bag and the face piece to be inhaled by the user.
Since the reactions of carbon dioxide and moisture with an oxygen-generating agent generate heat, the temperature of the oxygen-enriched gas discharged from the oxygen-generating agent canister to the breathing bag is raised. Therefore, one of the functions of the breathing bag is to hold the oxygen-enriched gas until it has cooled to a temperature suitable for inhalation by the user.
However, the oxygen-enriched gas discharged from the oxygen-generating agent canister is at a high temperature. In particular, with a self-contained closed-circuit oxygen-generating breathing apparatus which is adapted to contain a large amount of oxygen-generating agent so as to be suitable for a long period of use, the temperature of the gas discharged from the oxygen-generating agent canister could be 100.degree. C. or more near the end of that period. Conventional breathing bags of known breathing apparatuses have not been able to cool the oxygen-enriched gas to a temperature low enough for inhalation by a human being, and the user of such an apparatus often has difficulties in inhalation.
In general, the temperature of a gas which a human being does not find too hot to inhale and which can be inhaled without any pain is about 80.degree. C. when the gas is dry air, but, as the humidity of the gas rises, a human being would find it hot to inhale, even if the temperature of the gas is low. The moisture content of the exhalation gas exhaled by a human being is normally at saturation level. Therefore, a gas containing moisture at this level feels burningly hot when the gas is at a temperature of about 60.degree. C. or more, and reports have stated that a gas containing moisture at this level and having a temperature of about 55.degree. C. sometimes hurts a human being in violent motion. Considering these facts, since the gas to be inhaled has a certain moisture content, it is desired that the temperature of the gas does not exceed 52.degree. C. at most, in order to ensure easy breathing.
In order to solve the above-stated problems, various attempts have been made. For instance, attempts have been made to increase the volume of the breathing bag so as to lengthen the period during which the gas is cooled. However, this has achieved relatively little effect compared to the increase in volume of the bag, and has the disadvantages that the overall apparatus becomes bulky and is inconvenient to put on.